Can a class has a member array which will be dynamically allocated during runtime?

[runn]

Can a class has a member array which will be dynamically allocated during runtime? Such as

double* A=new double[n]

here n is an integer member variable and will be determined during runtime and the class will have the array *A allocated.

Thanks in advance.

[Banfa]

Yes

You might normally allocate the memory for the array in the constructor or some other member function of the class and then delete it in the destructor.

[runn]

The code is below:

#include <iostream>
using namespace std;
 
class MyClass
{
      public:
      int n;
      double* A;  // One of the problem is the declaration I think ...
      MyClass(int);
      virtual ~MyClass();
      void Print();
};
 
 
MyClass::MyClass(int _n)     
{
     n=_n;
     double* A=new double[n];
     if (!A) abort();
     A[0]=0.0;A[1]=10.0;A[2]=20.0;A[3]=30.0;
     cout <<A[0]<<endl;
     cout <<A[1]<<endl;
     cout <<A[2]<<endl;
     cout <<A[3]<<endl;
     cout <<""<<endl;
}
 
MyClass::~MyClass()     
{
     delete []A;
}
 
void MyClass::Print()
{
     cout <<A[0]<<endl;
     cout <<A[1]<<endl;
     cout <<A[2]<<endl;
     cout <<A[3]<<endl;
}
 
 
int main ()
{
 MyClass m(4);
 m.Print();
 return 0;
}

and the output of this code is:

• 0
  10
  20
  30
  
  1.98218e-317
  1.17289e-046
  0
  1.94765e-308

First 4 output comes from the constructor. Considering the last 4 output, I understand that allocated memory is local to the constructor member function.

Another problem is the declaration of A I think...

[Banfa]

That is because in the constructor rather than using the member variable A you declare a local variable A on the stack that is then lost when the function exits producing a memory leak.

Nothing is ever assigned to the member variable A.

[runn]

If I use the A in the member function in which the memory is allocated there is no problem. Every thing is OK. However I want to use it outside the member function also. For this purpose, When I make an declaration as

double* A[n]

compiler says:
invalid use of non-static data member `MyClass::n'

If I make n is static, there will be only one n while there are several MyClass objects. Each of them needs a n variable. Or I am wrong, not sure ...

[Banfa]

May I ask why you are not just using a vector?

[arnaudk]

However I want to use it outside the member function also.

So then why are you making a local redeclaraion (see reply #4)? Remove "double* " from line 18. Even better, use an initializer list:

MyClass::MyClass(int _n) : A(new int[_n])
{
     n=_n;
     if (!A) abort();
     A[0]=0.0;A[1]=10.0;A[2]=20.0;A[3]=30.0;
// etc
}
 

[runn]

May I ask why you are not just using a vector?

Thank you for answers. I am new to C++ and want to learn its basic rules at the beginning. Actually I want to write a matrix class and use it for scientific computing. They say vector class is not fast especially for small matrices. There may be a class from blitz which is fast. I can use it, but later.

So then why are you making a local redeclaraion (see reply #4)? Remove "double* " from line 18. Even better, use an initializer list

Because I didnt know how to allocate memory from inside the constructor. Thank you for pointing out initializer list. The code works now.